A system of subsea installations is commonly connected for the supply of power and for the communication of data by way of an umbilical connection to an installation which may be onshore or may be a fixed or floating structure offshore. The umbilical connection may be very long (such as tens of kilometers) and may by means of multiple taps provide power to or data communication with a multiplicity of subsea installations. It is known to convey control and monitoring data by means of various transmission protocols (such as TCP/IP or a token bus protocol) and it is also known to convey data signals employing coding such as orthogonal frequency division multiplexing (OFDM) employing, for example, carrier frequencies in the range from 3 kHz to 45 kHz. It is also known to superimpose data signals on power lines, using at each end of a power line (such as a power line in an umbilical) a modem which is coupled to the power line by way of an appropriate combiner or separator as appropriate.
Owing to the harsh and occasionally hazardous conditions in which subsea installations operate it is desirable to provide redundancy in at least one and preferably both the communication of data to and from subsea installations and the supply of power to them. Thus control of the installations can be maintained at least temporarily despite the failure of, for example, a single communication link or a single power supply module.
However, it is desirable to provide in a subsea installation a fail-safe system in which a device such as an actuator is automatically put into a predetermined condition either in the event of failure of an electrical link to the installation or by deliberate action of a controller. The predetermined condition is usually one in which a control valve is closed, so as for example to prevent flow of production fluids. An actuator for this purpose may be electrically driven to an operative state (e.g. one in which a valve is opened) and require the continuous presence of an electrical signal to remain in that state; if this signal is interrupted the actuator immediately reverts to a predetermined (‘fail-safe’) condition. Examples of such actuators are disclosed in our international patent application published as WO2012/123694. In one example, a valve spindle is driven against the force of a restoring spring and latched in the operative state; the latch is electromagnetically maintained unless the maintaining signal is interrupted, whereupon the latch releases and the mechanical spring immediate drives the spindle to the fail-safe state. However, other forms of actuators may be used.
The provision of a safety signal, i.e. a signal of which the loss or substantial diminution (herein called ‘inactive’) will cause a device such as an actuator to enter the fail-safe state may be at least partly incompatible with a communication or power system that provides redundancy. Moreover, although the use of existing power and/or communication lines (herein called generically ‘transmission lines’) would be convenient for safety signals there is a problem of avoiding cross-talk either between safety signals and non-safety signals or between safety signals for different installations, especially if the safety signals are in a modulated format and have therefore a substantial bandwidth. It is important for safety signals to be specific to a particular installation or device and therefore any possible confusion between them should be avoided.